Correlations Between the Microstructure and Fatigue Life of Near-Eutectic Sn-Ag-Cu Pb-Free Solders

Abstract

Relationships between the microstructure of near-eutectic Sn-Ag-Cu Pb-free solder joints and room-temperature fatigue lifetimes were studied. Correlations between the lifetimes of single Sn grained, SAC205 solder joints with the orientation of the Sn grain, and with differences in Ag 3 Sn and Cu 6 Sn 5 precipitate microstructures were sought. Correlations between the number of Sn grains and fatigue life were observed. Surprisingly, it was found that Ag 3 Sn precipitates were highly segregated from Cu 6 Sn 5 precipitates on a length scale of approximately 20 lm. Furthermore, large (factor of two) variations of the Sn dendrite arm size were observed within given samples. Such variations in values of dendrite arm size within a single sample were much larger than observed variations of this parameter between individual samples. Few significant differences were observed in the average size of precipitates in different samples. Although effects of average precipitate microstructure on lifetimes were not clearly delineated, one sample showed an anomalously high number of the smallest size (30 nm to 50 nm) Ag 3 Sn precipitates, and this sample also exhibited a much longer lifetime than all the other samples. Thus, some evidence was presented that samples of particular orientations and precipitate microstructures can exhibit anomalous fatigue lifetimes.

Dependence of Sn Grain Morphology of Sn-Ag-Cu Solder on Solidification Temperature

“…1) hypothesizes that the nucleus is a sixfold, multiply twinned b-Sn structure. [10][11][12][13] In fact, {101} twinning occurs in Sn with a twinning angle of 57.2°, consistent with such sixfold twinning (twinning is also commonly observed in Sn during deformation). The model maintains that a small hexagonal cluster of Sn atoms centered on a Ag or Cu atom forms in the melt (Fig.…”

“…The Sn grain morphologies of these Sn-Ag-Cu samples (Figs. 2a, [3][4][5][8][9][10][11][12][13][14][15] generally revealed only three main Sn grain orientations. Furthermore, these three unique Sn orientations were each rotated 60°around a common h010i axis.…”

Influence of bismuth on the solidification of Sn-0.7Cu-0.05Ni-xBi/Cu joints

“…Note that this is significantly different to joints made with Ag-containing solders on Cu substrates (e.g. SAC or SnAg solders), where there is usually only a single nucleation event and either single grain, cyclically twinned 'beachball', or interlaced twinned structures [52,[63][64][65][66][67] and no reproducible grain orientation relative to the substrate [68]. In the case of Sn-0.7Cu-0.05Ni-xBi (x≤5) solders, this type of βSn grain texture where the c-axis of the βSn crystal is oriented horizontally might be favourable for retarding electromigration that occurs fastest along the c-direction in the βSn crystal [69,70].…”

Thermal Fatigue Evaluation of Pb-Free Solder Joints: Results, Lessons Learned, and Future Trends

“…There are limited publications that show the quantification of precipitates in a careful manner and correlate them to the mechanical properties of solder joints. 48,49 However, no data exist to correlate such measurements to the ATC performance of solder joints.…”